AQ-00275-000, Rev. 3
21
Theory of Operation
During the traditional measurement of sample absorption by a spectrophotometer, the relationship
between absorptance and transmittance of the sample beam is described by the Kirchoff equation:
where:
a
λ
= fractional absorption and
τ
λ
= fractional transmittance.
When measuring the absorptance of a sample in the sample compartment, the detector signal of the
spectrophotometer represents the portion of the sample beam that is not absorbed or scattered by
the sample.
When using the DRA-CA-3300 accessory, it is convenient to use the Kirchoff relationship:
where
ρ
λ
= spectral reflectance of the sample beam at the sample surface and at a specific wave-
length.
During reflectance measurements, the reflected component of the sample beam is collected by the
integrating sphere and detected by the sphere detector. The detector signal represents the part of
the sample beam that is not transmitted and not absorbed by the sample substance.
Double Beam Spectroscopy
In a double-beam, ratio-recording spectrophotometer, the measurement of reflectance factor and
transmittance involves the performance of a Baseline Correction based upon values recorded in an
Uncorrected Baseline. Baseline Correction is used to compensate for changes in sphere efficiency
due to the introduction of the sample into the system and for any imbalance in the energy of the
sample and reference beams. This correction is performed automatically by the instrument. The
theoretical basis for Baseline Correction, as it applies to simple reflectance and transmittance mea-
surements, is explained briefly in this section.
Radiation from the instrument illumination sources is split into two different beams: the sample
beam and the reference beam. Each beam is interrupted periodically by means of an optical chop-
per such that the integrating sphere is illuminated alternately by the two beams. At any given wave-
length, the instrument records the ratio of the signal produced by the detector when the sphere is
illuminated by the sample beam to that when the sphere is illuminated by the reference beam.
Therefore, when an Uncorrected Baseline is performed, the value B recorded by the instrument
may be expressed as:
a
λ
τ
λ
+
1
,
=
a
λ
τ
λ
ρ
λ
+
+
1
,
=